Monthly Archives: January 2016

Today I’ve been reviewing the ECMA-334 C# specification, and in particular the section about class instance constructors.

I was struck by this piece in a clause about default constructors:

If a class contains no instance constructor declarations, a default instance constructor is automatically provided. That default constructor simply invokes the parameterless constructor of the direct base class.

I believe this to be incorrect, and indeed it is, as shown here (in C# 6 code for brevity, despite this being the C# 5 spec that I’m reviewing; that’s irrelevant in this case):

Here the default constructor in Derived clearly doesn’t execute a parameterless constructor in Base because there is no parameterless constructor in Base. Instead, it executes the parameterized constructor, providing the default argument value.

So, I considered whether we could reword the standard to something like:

If a class contains no instance constructor declarations, a default instance constructor is automatically provided. That default constructor simply invokes a constructor of the direct base class as if the default constructor contained a constructor initializer of base().

But is that always the case? It turns out it’s not – at least not in Roslyn. There are more interesting optional parameters we can use than just int foo = 5. Let’s have a look:

When base() is explicitly specified, that source location is treated as the “caller” for caller member info attributes. When it’s implicit (including when there’s a default constructor), no source location is made available to the Base constructor.

This is somewhat compiler-specific – and I can imagine different results where the default constructor could specify a name but not source file or line number, and the declared constructor with an implicit call could specify the name and source file but no line number.

I would never suggest using this little tidbit of Roslyn implementation trivia, but it’s fun nonetheless…